In the manufacture of optical communication cables, two design types are most frequently employed that are generally referred to as "central-core" and "loose-tube" designs. In the central-core design, a number of optical fibers are contained within a tube, which is located at the center of the cable. This cable typically includes strength members that are positioned between the central core and an outer plastic jacket. By comparison, loose-tube cable designs typically include a number of relatively small tubes that are positioned around a central strength member, and each tube encloses a number of optical fibers. In the loose-tube cable design, the fiber-containing tubes are longitudinally stranded around the central member, which is to say that the tubes are rotated around the central member along the length of the cable. An example of such a loose-tube cable design is disclosed in U.S. Pat. No. 5,621,841 to Field.
From a manufacturing standpoint, the central-core design is advantageous because it allows the various components of the cable to be assembled into their ultimate cable form in a single step rather than two or three steps as in the loose tube design. From an efficiency standpoint, optical fiber ribbon (i.e., a planar array of optical fibers that are bonded together as a unit) is advantageous because many ribbons can be stacked on top of each other within a small space. Accordingly, central-core cables having stacked optical fiber ribbons are highly desirable.
Longitudinal stranding is used in the optical cable industry to avoid subjecting individual fibers to undue tensile or compressive stress that may occur when the cable is bent. Such stress not only modifies the transmission characteristics of a fiber but also leads to breakage in extreme situations. In this industry, two types of longitudinal stranding are known: "continuous" and "S-Z." Continuous stranding is a process in which one or more strands of material (e.g., optical fibers) are helically rotated in a single direction along the longitudinal axis of the cable; whereas in S-Z stranding, the direction of rotation periodically reverses. S-Z stranding is preferred because it achieves the benefits of longitudinal stranding without the need for heavy machinery to lift and rotate large reels of strand material and it removes limits on the length of component materials. (Heretofore, stranding a stack of optical fiber ribbons in a central-core cable has been undesirable because transmission loss is significantly increased. This increase is frequently referred to as "cabling" loss since it is wholly attributable to the installation of transmission media [i.e., the ribbon stack] in a cable.) Furthermore, S-Z stranding has been difficult to achieve because twisted material tends to unwind at the point of reversal owing to inherent physical forces (restorative forces) that are created when a relatively stiff body is twisted --much like a torsion spring. This difficulty is exacerbated when the stiffness is increased, such as by stacking and bonding fiber ribbons together in a single unit.
In the formation of stacked ribbons, there is a tendency for the ribbons to stick together during sheathing which can create microbending, resulting in increased losses in the cable. The addition of suitable lubricant between the ribbons has resulted in substantially eliminating such losses. However, it has been found that many lubricants, e.g., oils, can present additional problems, stemming from characteristics of the lubricant such as viscosity, surface tension, contact angle, and surface wetting. These characteristics can lead to inadequate lubricating of the ribbons with a consequent increase in losses. Accordingly, what is desired and what the industry presently lacks is a central-core optical cable having the individual fibers or the ribbons adequately lubricated with the lubricant having high viscosity, low surface tension, and a small contact angle.